RH EL3 on hundreds of physical and virtual boxes (vmware esx/gsx and server).  

I don't admin those machines so I can't comment too much on them.  I know 

that the plan is to move up to EL4 in the next few months.  The advantage of 

RH over Debian and the like is that *everything* works on RH.  And if you're 

running Oracle (or similar), RH is one of the few systems that will likely be 

certified.  Finally, if your admins are "Linux certified", it'll likely be 

for RH.